Self-inking stamp



FIG. 1 is a top, front and right side perspective view of a self-inking stamp showing the new design;

FIG. 2 is a right side elevation thereof; the left side elevation being a mirror image thereof;

FIG. 3 is a top plan thereof; and,

FIG. 4 is a front elevation thereof, the rear elevation being a mirror image thereof.

The broken lines illustrate portions of the self-inking stamp that form no part of the claimed design. 

CLAIM The ornamental design for a self-inking stamp, as shown and described. 